1. Field of the Invention
This invention relates to a radiation image read-out device for reading out the image information recorded in a stimulable phosphor by stimulating the same and measuring the light emitted therefrom upon stimulation, and more particularly to a device for reading out the diagnostic radiographic image information recorded in a stimulable phosphor which is able to store radiation energy and emits the stored energy upon stimulation by stimulating rays.
2. Description of the Prior Art
A stimulable phosphor stores a part of the energy of a radiation when it is exposed to the radiation like X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, ultraviolet rays and so forth. Then, upon stimulation thereof by stimulating rays the stimulable phosphor emits light the amount of which corresponds to the amount of energy stored therein.
It is known in the art to utilize the stimulable phosphor to once record an X-ray image of a human body or the like in a stimulable phosphor sheet, read out the recorded image with a photodetector by scanning the phosphor sheet by means of a laser beam or the like which stimulates the phosphor, and record the X-ray image on a recording medium like a photosensitive film by use of a recording light beam which is modulated by the image information read out by the photodetector. One of such devices is disclosed in U.S. Pat. No. 3,859,527.
In said patent a large mirror is used to reflect light emitted from a stimulable phosphor towards a photodetector. As the mirror is used a semi-transparent mirror which partly transmits the stimulating rays impinging upon the stimulable phosphor. The light emitted from the phosphor upon stimulation is partly reflected by the semi-transparent mirror which is inclined at 45.degree. with respect to the optical path of the stimulating rays and is accordingly directed in parallel to the surface of the phosphor. The reflected light is received and measured by a photodetector by way of a condenser lens.
The above described device has a disadvantage that the quality of the radiation image information useful for diagnosis is very poor. This is because the light emitted from the stimulable phosphor upon stimulation diverges to all directions and accordingly the very limited fraction of the light is sensed by the photodetector. This situation reduce badly the S/N ratio of the image. Particularly because it is impossible from the viewpoint of safety to expose the human body to a radiation of large dose, the amount of light emitted from the stimulable phosphor which is in proportion to the amount of radiation energy stored therein is inevitably small. Therefore, in order to read out the radiation image information with a high S/N ratio enough for diagnosis, it is necessary to prevent the stimulating rays from impinging upon the photodetector at first, and secondly to introduce as much light emitted from the phosphor as possible into the photodetector. In the above described device disclosed in said U.S. Patent, however, there is no specific solution of this problem and accordingly it is impossible to get the radiation image information which is practically useful for diagnosis.
In order to solve the above problem and improve the radiation image information read out device of this type, the instant inventors have suggested a method of enhancing the S/N ratio by selectively detecting only the light emitted from the phosphor. This was conducted by making the wavelength of the stimulating rays differ from that of the light emitted from the phosphor and using a light detecting means which only detects the light of the wavelength of the latter. As the light detecting means can be used for instance a photodetector which is only sensitive to the light of said wavelength or a filter located in front of the photodetector which only transmits the light of said wavelength. For instance, as the stimulating rays is used a red laser beam having a wavelength distribution in the range of 600 to 700 nm which is different from the wavelength of the light emitted from the phosphor that is 300 to 500 nm. The above suggestion made by the instant inventors is disclosed in a copending Japanese patent application No. 53(1978)-84741, corresponding to U.S. Pat. No. 4,258,264.
Further, it was also suggested by the instant inventors that a photodetector is located extremely close to the phosphor in order to enhance the light collection efficiency (Japanese patent application No. 53(1978)-122881). This method is, however, disadvantageous in that the scanning mechanism of the stimulating rays becomes complex.
Thus, it has still been very difficult to obtain the radiation image information which is practically useful for diagnosis.